ocaml: phase 2 references ref/!/:= (+6 tests, 189 total)

ref is a builtin boxing its arg in a one-element list. Prefix ! parses
to (:deref ...) and reads via (nth cell 0). := joins the binop
precedence table at level 1 right-assoc and mutates via set-nth!.
Closures share the underlying cell.

plans/ocaml-on-sx.md

@@ -153,7 +153,7 @@ SX CEK evaluator (both JS and OCaml hosts)
 - [x] `if`/`then`/`else`, `begin`/`end`, sequence `;`.
 - [x] Arithmetic, comparison, boolean ops, string `^`, `mod`.
 - [x] Unit `()` value; `ignore`.
-- [ ] References: `ref`, `!`, `:=`.
+- [x] References: `ref`, `!`, `:=`.
 - [ ] Mutable record fields.
 - [ ] `for i = lo to hi do ... done` loop; `while cond do ... done`.
 - [ ] `try`/`with` — maps to SX `guard`; `raise` via perform.
@@ -320,6 +320,12 @@ the "mother tongue" closure: OCaml → SX → OCaml. This means:
 
 _Newest first._
 
+- 2026-05-08 Phase 2 — references (`ref`/`!`/`:=`). `ref` is a builtin
+  that boxes its argument in a one-element list (the mutable cell);
+  prefix `!` parses to `(:deref EXPR)` and reads `(nth cell 0)`; `:=`
+  joins the precedence table at the lowest binop level (right-assoc) and
+  short-circuits in eval to mutate via `set-nth!`. Closures capture refs
+  by sharing the underlying list. 189/189 (+6).
 - 2026-05-08 Phase 3 — pattern matching evaluator + constructors (+18
   tests, 183 total). Constructor application: `(:app (:con NAME) arg)`
   builds a tagged list `(NAME …args)` with tuple args flattened (so